[7.4 rc fix] [doc] add optimizations for mysql to 6.x->7.4 guide

Fixes from master

It includes:

* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1193
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1196
* https://dev.gitlab.org/gitlab/gitlabhq/merge_requests/1195

See merge request !1198

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

We need to skip empty repositories when creating a backup. Before this
change, we were asking gollum-lib if the wiki contains any _pages_. Now
we ask gitlab_git if the repository contains _files_. This should
resolve gollum_lib Grit timeouts in the backup script.

Add mysql optimizations from 7.3-to-7.4.md.

Merge master into 7-4-stable

See merge request !1189

Fix user being blocked after oauth sign-in

See merge request !1190

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Issue respect single sign on

Adds regression test for #1677

See merge request !1188

verification for #1677

Since testing omniauth_callback_controller.rb is very difficult, the logic
is moved to the models

This is handled within the LDAP class

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Add a few items to changelog

See merge request !1187

Issue respect single sign on

Fixes #1677 by making sure the per-build user is saved as well. Otherwise the user is passed to devise, and the user is saved afterwards, creating a successful signup.

See merge request !1186

Because devise will trigger a save, allowing unsaved users to login, behaviour had changed.
The current implementation returns a pre-build user, which can be saved without errors.

Reported in #1677

Fix link to githost.io

Documentation pointed to wrong gitlab-ci project.

See merge request !222

Note omnibus in installation

See merge request !1184

Improve locked merge request UI

It allows us easily detect when MR get into lock state

Fixes UI part of #1674

See merge request !1183

No red builds policy

Related to gitlab/organization#63

See merge request !1182

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Conflicts:
	doc/workflow/README.md

same rendering for note diff as for usual diff

Changelog: added information about sorting users

See merge request !1175

Notifications documentation

Fixes #1583

See merge request !1177